If you edit any existing parts you will need to give them a new letter - you cannot reuse the old one. For example if you had A and B last year and have changed the areas of those splits they will need to become C and D. You might wish to merge the parts as above and then redo the splits.
For more help on SPS call our Adviceline team on 01594 545022 or email us at support@farmplan.co.uk.